Method of making ultra-fine ammonium perchlorate particles

ABSTRACT

A method of making ultra-fine ammonium perchlorate particles by spraying an aqueous ammonium perchlorate solution containing a surface active agent onto a moving film of a refrigerated organic liquid to form fine frozen droplets in said film and recovering fine ammonium perchlorate particles from the frozen droplets by freeze drying. The product particles have weight mean diameters of less than 1 micron and are especially useful in the manufacture of solid rocket propellants.

[ June 25, 1974 METHOD OF MAKING ULTRA-FINE AMMONIUM PERCHLORATEPARTICLES [75] Inventors: Rudy E. Rogers; James L. Murphy,

Jr., both of Huntsville, Ala.

[73] Assignee: Thiokol Chemical Corporation,

Bristol, Pa.

[22] Filed: Dec. 20, 1972 [21] Appl. No.: 316,677

[52] US. Cl 23/302, 149/76, 264/3 C, 264/14, 62/58, 423/476 [51] Int. ClCOld 1/30 [58] Field of Search 149/76; 423/476; 264/3 C, 264/3 E, 14,298; 23/302, 294; 62/58 3,419,899 12/1968 Tufts et a1 23/302 3,498,7593/1970 Kralik 23/302 3,551,533 12/1970 Monforte 264/14 PrimaryExaminer-Stephen J. Lechert, Jr.

[5 7] ABSTRACT A method of making ultra-fine ammonium perchlorateparticles by spraying an aqueous ammonium perchlorate solutioncontaining a surface active agent onto a moving film of a refrigeratedorganic liquid to form fine frozen droplets in said film and recoveringfine ammonium perchlorate particles from the frozen droplets by freezedrying. The product particles have weight mean diameters of less than 1micron and are especially useful in the manufacture of solid rocketpropellants.

6 Claims, 1 Drawing Figure METHOD OF MAKING ULTRA-FINE AMMONIUMPERCHLORATE PARTICLES This invention relates to the manufacture ofammonium perchlorate of a particle size adapted to be used in rocketpropellants and more particularly to a novel method of making ammoniumperchlorate in the form of ultra-fine particles.

Most solid rocket propellants are principally composed'of a polymericfuel-binder having a finely divided oxidizer, usually ammoniumperchlorate, dispersed therein. The oxidizer, a curing agent for thepolymeric binder, and certain special purpose ingredients known in theart are dispersed in the binder while the latter is in viscous fluidform, and the resulting mixture is cast in situ in a rocket motor casingat a temperature sufficient to convert the binder to elastomeric form.

It is known that the efficiency and rate of combustion of a solidpropellant can be maximized by increasing the area of contact betweenthe oxidizer and other irihazard and numerous precautions must be takento minimize the risk of explosion. Moreover, the risk of explosion cannever be completely eliminated. Also there is a tendency for theammonium perchlorate to become contaminated by material removed from thegrinder and grinding media. In addition, grinding is a relativelyexpensive procedure and it has not been found possible to reduce theparticle size of the oxidizer by grinding to the extent that isdesirable for highly efficient propellant combustion.

Because of the foregoing disadvantages various proposals have previouslybeen made for producing finely particulate oxidizers other than bygrinding. Thus US. Pat. No. 3,452,445 discloses a process for makingfine particles of ammonium perchlorate by quick freezing of aqueousammonium perchlorate. Freezing of the aqueous solution is effected byrapidly rotating a fiask containing the solution in an acetone-dry icebath. The ice thus formed is then sublimed under a reduced presof lessthan one micron. It is another object of the invention to provide aprocess for making ammonium perchlorate which eliminates the explosionhazard involved in the grinding operation referred to above. it isanother object of the invention to provide a process for makingultra-fine ammonium perchlorate of high purity at a low unit cost. It isstill another object of the invention to provide such a process than canbe readily automated. It is a still further object of the invention toprovide a process wherein for any given production rate the quantity ofmaterial being processed is relatively small. Other objects of theinvention will be in part obvious and in part pointed out hereafter.

sure to recover ammonium perchlorate particles having an average size of1.7 to 2.1 microns.

US. Pat. No. 3,222,231 to Markels Jr. et. al. discloses a processwherein a saturated aqueous solution of ammonium perchlorate at atemperature of 95 C. is simultaneously agitated and subjected toultra-sonic vibrations over a period of several hours. As the solutioncools,.ammonium perchlorate crystals precipitate and are'subsequentlyremoved from the solution, washed with acetone and ether, and dried. Theproduct crystals had a particle size within the range 5 to 350 microns.

While such processes avoid the explosion hazards involved in mechanicalgrinding, the size of the ammonium perchlorate particles they produce isof the same As conducive to a clearer understanding of the presentinvention, it may be pointed out that in wet processes for producingammonium perchlorate, as in most crystallization processes, thecrystallization takes place in two stages, namely, crystal formation andcrystal growth. In cases such as the present one where very smallparticle sizes are desired, it is necessary to provide either some meansof inhibiting crystal growth of the particles as they are first formedor, if the particles are permitted to grow, some means of breaking themup.

In accordance with a preferred embodiment of the present process,ultra-fine particles of ammonium perchlorate are produced by preparingan aqueous solution of ammonium perchlorate containing a small amount ofa crystal growth inhibitor and spraying the solution on a thin, movingfilm of a water-immiscible, volatile organic liquid at a temperaturebelow the freezing point of the aqueous solution to produce a slurry ofice crystals and ultra-fine ammonium perchlorate particles. Byspray-freezing the ammonium perchlorate in a moving film of arefrigerated immiscible liquid, extremely rapid freezing is achieved toyield very small crystals of ammonium perchlorate. Growth of these smallcrystals is inhibited by the presence of the crystal growth inhibitorwhich, as pointed out more fully below, is a further active agent. Ithas been found that the surface active agent, even when present in arelatively small amount, appears to be effective in inhibiting crystalgrowth in the slurry, possibly by coating the ammonium perchlorateparticles, and thus stabilizes the particle size of the solid phase ofthe slurry produced in the spray-freezing step of the present method.

The recover the ultra-fine ammonium perchlorate particles from theslurry as thus formed, the organic liquid is vaporized from the slurryat a low enough temperature to avoid melting of the ice crystals toleave a mixture of the ice crystals and ammonium perchlorate particles.Separation of the ice crystals from the ammonium perchlorate particlesis effected by freeze-drying. The mixture is maintained at a temperatureof say 40 to+l0F., preferably about -20 lF., and at a reduced pressure,say 1 to micron of mercury absolute, to cause the ice crystals tosublime. The residue comprising a porous friable mass of ammoniumperchlorate particle aggregates can be broken up into ultra-fineparticles in any of several ways. Thus if the product ammoniumperchlorate is to be used in a propellant of the type described above, acurable liquid organic fuelbinder is cured to an elastomer, the residuefrom the freeze-drying step can be directly incorporated in thepropellant mixture prior to curing, and the usual mixing of the viscouspropellant composition prior to curing will serve to disperse theultra-fine ammonium perchloduce a dispersion of the ultra-fine ammoniumperchlorate particles. In order to achieve a stable submicron particlesize, a suitable coating agent, e.g., a carboxylterminated orhydroxyl-terminated liquid hydrocarbon polymer, is desirablyincorporated in the organic liquid dispersion of ammonium perchlorate.

Once the ultra-fine ammonium perchlorate particles have been formed inthe spray-freezing step of the present process, they should desirably bekept out of contact with atmospheric moisture, since such moisture tendsto induce crystal growth, particluarly after the particles have beenfreeze dried.

It has been found that by carrying out the present method using aspray-freezing solution containing a surface active agent of a typedescribed more fully below in an amount of say 0.5 to percent by weightof the ammonium perchlorate used, a product having an average particlesize of 0.5 to 1.0 micron can be readily obtained, whereas asubstantially larger particle size is obtained when the surface activeagent is omitted from the spray-freezing solution.

The concentration of ammonium perchlorate in the aqueous solution usedin the spray-freezing step of the process may vary over a relativelywide range, say 1 percent to 33 percent by weight. The preferredconcentration is 5 percent to 20 percent by weight. Since the particlesize produced is to some extent a function of the size of the dropletssprayed on the refrigerated liquid film, a spray capable of producingdroplets of a diameter less than microns is preferably used.

The immiscible organic liquid into which the solution issprayed shoulddesirably have a relatively low freezing point, i.e., 70F. or lower, andshould be sufficiently volatile to permit it to be rapidly evaporatedfrom the slurry formed in the spray-freezing step. The preferred organicliquids are dichlorodifluoromethane (Freon 12) and trichloroethylene.Liquid nitrogen may also be used.

As indicated above, other factors being maintained constant, the use ofa small amount of a surface active agent in the spray solution resultsin a substantial reduction in average particle size. Suitable exemplarysurface active agents for use in the present process are given in TableI.

TABLE I Trade Designation Strodex PK Strodex SV 8 Surface Active AgentPotassium salt of polyphosphoric ester acid Concentrated ionized complexmulti-carbon alcohol-potassium neutralized Alkyl sulfosuccinate of theformula H I cal-000m NEOaB-(B--CHr-C O NH-COONa OONa (431a) elkylMonawet SN0 TABLE I Continued Surface Active Agent Trade DesignationNa-Z-ethylhexyl sulfate Emcol DS-lO Alcohol sulfate, amine salt DuponolEP High molecular weight carbohydrate polymer composed primarily ofgalactomannan with borax added to produce cross-linking J B. Polymer Thespray-freezing step of the present process can be carried out in anysuitable apparatus capable of producing a thin film of cooled liquid inwhich the sprayed solution is insoluble. One suitable type of apparatusis shown in the accompanying drawing. Referring to the drawing, thefreezing apparatus there shown comprises a vertically arranged tank 10which may be of cylindrical cross section and in which a mixing weir 12is centrally located. Mixing weir 12 may also be of generaly cylindricalconfiguration and coaxial with tank 10. The weir flares outwardelliptically at its upper end 13 until the'top edge of its wall isessentially horizontal. This upper end 13 rests on and is attached to anupper annular flange 14 that extends inwardly from the top of an innercylindrical wall 16. Wall 16, of a somewhat smaller diameter than tank10, is supported at its lower end by a lower annular flange 18 thatextends inwardly from the interior surface of tank 10. The top of tank10 is enclosed by a cover 22 that is circumferentially supported by anexternal upper tank flange 24. A sight port 26 is provided in'tank lid22 to permit operators of the apparatus to view the spray-freezingprocess.

An injector assembly 28 is positioned through the center of tank lid 22so that a spray nozzle 30 forming part thereof extends below the topedge of weir 12. The injector assembly includes a hot water jacketedsection 32 through which hot water flows to prevent precrystallizationof the solution flowing to and through nozzle 30.

The ammonium perchlorate solution to be crystallized is contained in asolution storage tank 42 that is pressurized by gas'stored in apressurization tank 44. The conduit 45, between tanks 44 and 42,contains a pressurization valve 46 that is used to control thepressurization of tank .42. A solution flow conduit 48 connects tank 42to a pipe 31 of the injector assembly 28.

Adjustment of solution flow rate is controlled by a solu-' tion flowvalve 50 that is located in conduit 48.

Located between the inner cylindrical wall 16 and the wall of tank 10there is a heat exchanger 52 comprising a refrigeration coil 54 that issupplied with a refrigerant through a refrigerant input conduit 56. Therefrigerant is cooled by a refrigeration unit 58 that supplies conduit56. The bottom turn of coil 54 is connected to the bottom turn of alower refrigeration coil 60 by a refrigeration coil connecting conduit62. Coil 60 is positioned in the lower portion 64 of tank 10 and iscovered by the coolant liquid that is maintained at a level 66 above thecoil as shown. The uppermost turn of coil 60 is connected to arefrigerant return conduit 68 that directs the flow of the refrigerantback to refrigeration unit 58.

A coolant exitport 70 is located at the bottom of tank portion 64 and isin communication with a coolant return conduit 72. A cryogenic ballvalve 74 is located in conduit 72 to provide appropriate control of thecoolant flow. A second cryogenic ball valve 76 is connected to thecoolant drain 78 that branches from a T- connection 79 in conduit 72.Conduit 7 is connected to the input of a coolant circulating pump 80that directs the coolant through a coolant feed conduit 82 to the bottomof heat exchanger 52. The coolant then flows upwardly around the coil 54which serves to further refrigerate it, and flows over the upper end 13of weir 12, forming a thin film on the inside surface of the weir.

As the coolant film flows downwardly on the inner surface of weir 12, itis struck by the spray emitted from nozzle 30. This contact with thecoolant immediately freezes the spray droplets, thereby forming a slurryof the frozen droplets in the coolant. This slurry then continues toflow down the inner surface of weir 12 and drops into a basket 86positioned in a basket support 88. Basket 86 is formed from a tightlywoven screen, for example, 325 mesh, which holds the frozen droplets butpermits passage of the coolant to the lower portion 64 of tank 10. Thebasket support 88 extends through the wall of tank and an insulation andvapor barrier 90, which encloses the entire tank.

in operation, a constant, predetermined flow of the coolant liquid overthe upper circumferential edge of weir 12 is established. The sight port26 can be used to view this flow to ensure that the coolant only forms athin film on the inner surface of the weir. As previously described, thedroplets of solution discharged by spray head 30 impinge upon theflowing refrigerated liquid and are immediately frozen to form a fineparticle size slurry that flows from the bottom of weir 12 into thebasket 86. Most of the coolant liquid drains through the basket into thelower portion 64 of tank 10 and is cooled and recirculated as describedabove.

When a desired amount of thickened slurry has collected in basket 86,the accumulated slurry is removed from the spray-freezer and residualcoolant liquid is separated from the frozen particles by evaporation.This step of the process can be carried out in any suitable apparatus.It is only necessary that the coolant be evaporated in a substantiallymoisture-free environment and at a temperature low enough to preventmelting of the frozen particles of ammonium perchlorate solution.

When the frozen solution particles are essentially free from residualcoolant, they are transferred to a freeze drier, which may be a known,commercially available type of equipment, for example, a model 41 RePPsublimator manufactured by The Virtis Co., Inc. Such freeze drierscommonly comprise an evacuated chamber having shelves adapted to supporttrays containing the material to be freeze dried and a condenser forcondensing the water vapor formed by sublimation of the ice crystals.Desirably the shelves are heated to increase the sublimation rate.

Upon completion of the freeze drying step, the solid residue comprisesessentially a friable aggregate of very fine ammonium perchlorateparticles which, if they are to be stored, are packaged in hermeticallysealed containers. As indicated above, if the ammonium perchlorateproduct is to be used in solid propellant manufacture of the type inwhich an inorganic oxidizer is mixed with a liquid curable organicpolymeric fuelbinder, the product may be added directly to aconventional propellant mix, and the normal mixing operation willdisperse the fine particles of the product. Alternatively, the productmay be dispersed in a liquid non-solvent with vigorous mixing and/orultra-sonic treatment to produce a dispersion of the fine ammoniumperchlorate particles.

In order to point out more fully the nature of the present invention,the following specific examples are given of illustrative embodiments ofthe process of the invention. Example 1 A solution of 392 grams ofammonium perchlorate in 3,600 ml. of water was prepared and 3.93 gramsof a potassium salt of polyphosphoric ester'acid (Strodex PK- was addedthereto as a surface active agent. Thus the solution contained about 10percent by weight of ammonium perchlorate and the surface active agentconstituted about i percent by weight of the ammonium perchlorate.

The prepared solution was pressurized in a closed stainless steel vesselto 900 p.s.i.g. and sprayed from the vessel through a 1212 (SprayEngineering Co.) hydraulic nozzle having an orifice of 0.012 inchdiameter. The nozzle was wrapped with copper heating coils to preventfreezing of the solution before spraying. The temperature of thesolution at the outlet of the spray was 55F.

The sprayed solution was frozen in apparatus of the type described aboveand shown in the accompanying drawing using a refrigerated film of Freonl2 chilled to 62F. by circulation over refrigerated coils. The stream ofFreon 12 with entrained droplets of frozen solution was fed to a325-mesh screen which separated the frozen particles.

The separated slush of frozen particles was stored in a low temperaturechamber at about 30F. until the Freon 12 had evaporated, after which thefrozen material was transferred to pre-cooled trays-of a freeze dryer. Avacuum of 50 microns was established in the freeze drying chamber and ashelf temperature of F. was established on the shelves supporting thetrays containing the frozen material to vacuum sublime the ice. After 24hours, the material was sufficiently dry and the dry ammoniumperchlorate powder was packaged and sealed. The resulting particles hada weight means diameter of 0.47 microns. The total residual moisturecontent of the ammonium perchlorate was 0.061 percent by weight.

Example 2 A solution of 529.4 grams of ammonium perchlorate in 3.000 ml.of water was prepared and 5.3 grams of Strodex PK-90 was dissolvedtherein as a surface active agent. The solution was spray frozen andfreeze dried as in Example I.

The weight mean diameter of the freeze-dried particles was 0.535 micronsand their water content was 0.087 percent.

Example 3 A solution to be freeze-dried was prepared to contain 10percent by weight of ammonium perchlorate in 90 percent water. To thesolution there was added as a surfactant 1 percent by weight of theammonium perchlorate of alkylamine sulfosuccinate (Emcol 4,200).

The prepared solution was pressurized in a closed stainless steel vesselto 800-1000 p.s.i.g. and sprayed into a flowing film of refrigeratedliquid as in Example 1. However, trichloro-ethylene at a temperature of55 to 75F. was used in place of the Freon 112 of Example l. The flowrate of the trichloroethylene over the weir plate was about 6 gal/min.The stream of trichloro-ethylerle was entrained frozen droplets of thesolution was fed to a 325-mesh screen to separate the frozen particles.

The frozen material was transferred to trays on the shelves of a freezedryer, and vacuum established in the drying chamber. Thetrichloroethylene from the frozen slush collected in the bottom of thedrying chamber where it was drained from the system periodically.

After the trichloroethylene was removed, heat was supwas used as asurfactant in place of the Emcol 4,200.

The weight means diameter of the ammonium perchlorate particles was 0.88microns and their water content was 0.13 percent. Example The procedureof Example 3 was followed except that a sulfosuccinate surface activeagent (Monawet SNO) was used in place of the Emcol 4,200.

The weight mean diameter of the ammonium perchlorate particles was 0.84microns and their water content was 0.15 percent.

From the foregoing description it should be apparent that the presentinvention provides a process capable of achieving the objectives setforth at the beginning of the. specification. By using an aqueoussolution of ammoniurn perchlorate containing the disclosed surfaceactive agents, product particles having weight mean diameters of lessthan one micron can be consistently obtained. The surface active agentsinhibit crystal growth not only in the spray freezing step of theprocess, but also in the subsequent process steps such as the coolantevaporation and freeze drying steps, as well as in the subsequentstorage of the material.

It is, of course, to be understood that the foregoing examples areintended to be illustrative and that numerous changes can be made in theingredients, proportions and conditions disclosed without departing fromthe spirit of the invention as defined in the appended claims.

We claim:

1. The method of making ultra-fine ammonium perchlorate particles whichcomprises preparing an aqueous solution of ammonium perchloratecontaining a surface active agent, spraying said solution on a thin,moving film of a water-immiscible, volatile, organic liquid at atemperature below the freezing point of said solution to form aslurry'of ice crystals and ultra-fine ammonium perchlorate particles,separating said organic liquid from said slurry, heating the resultingmixture at a reduced pressure to cause said ice crystals to sublime andrecovering said ultra-fine ammonium perchlorate particles.

2. A method according to claim 1 wherein said aqueous solution containsfrom 1 percent to 33 percent by weight of ammonium perchlorate.

3. A method according to claim 1 wherein said aqueous solution containsfrom 5 percent to 20 percent by weight of ammonium perchlorate.

4. A method according to claim 1 wherein said aqueous solution containsfrom 0.5 to 5 percent of said surface active agent based on the weightof ammonium perchlorate therein.

5. A method according to claim 1 wherein said organic liquid is selectedfrom dichlorodifluoromethane and trichloro-ethylene.

6. A method according to claim 1 wherein said organic liquid isseparated from said slurry by evaporation.

2. A method according to claim 1 wherein said aqueous solution containsfrom 1 percent to 33 percent by weight of ammonium perchlorate.
 3. Amethod according to claim 1 wherein said aqueous solution contains from5 percent to 20 percent by weight of ammonium perchlorate.
 4. A methodaccording to claim 1 wherein said aqueous solution contains from 0.5 to5 percent of said surface active agent based on the weight of ammoniumperchlorate therein.
 5. A method according to claim 1 wherein saidorganic liquid is selected from dichlorodifluoromethane andtrichloro-ethylene.
 6. A method according to claim 1 wherein saidorganic liquid is separated from said slurry by evaporation.